1. Field of the Invention
The present invention relates to boilers for the production of hot water or steam.
2. Description of the Prior Art
Known boilers consist of a principal enclosure limited by a watertight wall for containing water, water supply piping means to introduce the water into the principal enclosure through at least one water inlet, and water extraction piping means to extract the water and/or the steam from the principal enclosure through at least one outlet. Electric resistor means, connectable to an external source of electric energy, are disposed inside the enclosure or in its walls in order to heat the water contained in the principal enclosure.
When such known boilers are used, the mineral elements dissolved in the water tend to be deposited on the walls of the boiler, forming a film of scale. This film progressively constitutes a thermally insulating wall between the water to be heated and the heating electric resistor means, thereby causing untimely overheating of the resistors and reducing the amount of hot water or steam produced for the same consumption of electricity.
To overcome these drawbacks, it is necessary to clean the boiler periodically, e.g. by introducing a sufficient quantity of dilute acid, for example vinegar, in order to dissolve the film of scale. Other methods consist in treating the water before it is introduced into the boiler.
However, all these known methods incur running costs which are by no means negligible and require constant supervision to ensure timely intervention.
In practice, cleaning operations are not carried out with sufficient regularity, and are dependent on the care shown by the user with regard to the maintenance of the boiler.
Boiler structures, such as those described in document DE-A-2 611 851 or document U.S. Pat. No. 4,032,748, are also known. In these structures the deposits of scale on the heating element are broken up through the effect of the variable thermal expansion of the heating element wall. The efficiency of these structures proves insufficient.
Piping and heat exchange structures between two liquids have been known for a long time, through document DE-C-606 028. In these structures the heat exchange wall separating the two liquids allows two positions of equilibrium. The passage from one position of equilibrium to another is brought about by applying a pressure to the outer liquid. Although this solution has been known for a long time, it has not been applied to boilers, and indeed it is not directly applicable.